Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, will reduce the mass and can reduce the aerodynamic frontal area of a vehicle.
Referring to FIG. 1, a turbocharger (10) uses the exhaust flow from the engine exhaust manifold to drive a turbine wheel (12), which is located in a turbine housing (14) to form a turbine stage (16). The energy extracted by turbine wheel (12) is translated into a rotating motion which then drives a compressor wheel (18), which is located in a compressor housing (20), to form a compressor stage (22). The compressor wheel (18) draws air into the turbocharger (10), compresses this air, and delivers it to the intake side of an engine (not shown).
Turbocharger compressors include three main components: the compressor wheel (18), a diffuser (24), and the housing (20). The compressor stage (22) works by drawing air axially into an inlet (25) of the compressor housing (20) and accelerating the air to high tangential and radial velocity through the rotational speed of the compressor wheel (18). This air, which still has substantial kinetic energy, is expelled in a radial direction into the diffuser (24). The diffuser (24) recovers as much of the kinetic energy as possible, by translating the high velocity of the air into increased air pressure and temperature. The volute (26) then collects the air and slows it down before it reaches the compressor exit, further recovering velocity into pressure.
The operating behavior of a compressor within a turbocharger may be graphically illustrated by a “compressor map” associated with the turbocharger. FIG. 2 depicts an example of a map for a compressor stage. The Y axis (28) is the pressure ratio, that is, the ratio of air pressure out of the compressor to the air pressure into the compressor. The X axis (30) is the mass flow rate (here in Kg/sec) into the compressor stage. In general, the operating behavior of a compressor wheel is limited on the left side of the compressor map by a surge line (32) and on the right side of the compressor map by a choke line (34). The generally horizontal lines (36) are lines of equal turbocharger speed. The compressor map can include an engine operating line (38), which shows, for a given set of operating conditions, where the map fits the air requirements of the engine operating regime.
The surge line (32) is a test-generated line. At each speed line, the surge point is detected, noted, and then interpolated for the entire map. The surge condition can move with installation conditions so it must be tested for each set of installation parameters.
The surge line basically represents “stalling” of the airflow at the compressor inlet. As air passes through the air channels between the blades of the compressor wheel, boundary layers build up on the blade surfaces. These low momentum masses of air are considered a blockage and loss generators. When too small a volume flow and too high of an adverse pressure gradient occurs, the boundary layer can no longer adhere to the suction side of the blade. When the boundary layer separates from the blade, stall and reversed flow occurs. Stall will continue until a stable pressure ratio, by positive volumetric flow rate, is established. However, when the pressure builds up again, the cycle will repeat. This flow instability continues at a substantially fixed frequency, and the resulting behavior is known as surging. The phenomenon of surge is quite violent, causing rapid changes of speed and load reversals in the turbocharger, the result of which is often destruction of the turbocharger. The turbocharger must be kept out of this operating range.
One known method that is employed to avoid surge involves bypassing air from the high pressure side of the compressor (i.e., the volute or compressor exit) and resupplying it to the compressor inlet. As a result, the mass flow of air into the compressor is increased and the operational point of the compressor stage moves away from the surge line. In the compressor map example of FIG. 2, the operational point would move to the right on the map. While such a bypass system can be helpful in avoiding surge, the energy of such bypass air is wasted.
Thus, there is a need for systems and methods to recover energy from charge air used during surge avoidance operation of the compressor stage.